The present invention relates generally to an apparatus for controlling the intake air amount of an internal combustion engine, and more particularly to a stepping motor applicable particularly, but not exclusively, to systems for driving an air intake device such as throttle valve to introduce air into an internal combustion engine.
Generally, in internal combustion engines for motor vehicles, a throttle valve is provided in an air intake passage and operatively coupled to an accelerating pedal of the motor vehicle and the engine output is controllable by the opening and closing operation of the throttle valve made in connection with vehicle driver's depressing operation of the accelerating pedal. As a system for preventing a vehicle slip possible when the vehicle is runing on a low running resistance surface such as a frozen road surface is known a so-called traction control system, in which wheel slipping is detected on the basis of signals from wheel speed sensors attached to driven and non-driven wheels of the vehicle and, in response to the detection, the throttle valve is controlled to be operated to the closing direction. This may repress the engine output to enhance the gripping force of the wheel, resulting in prevention of the wheel slipping.
The traction control system generally requires a means for driving the throttle valve independently of the accelerating pedal as described in Japanese Patent Provisional Publication No. 57-116140, for example, where the throttle valve is operatively coupled to a drive motor which is in turn controlled in accordance with the depressing amount of the accelerating pedal, the degree of the wheel slipping and so on. While successfully performing the optimal control of the throttle valve and making easy the constant-speed control of the vehicle, such an apparatus would suffer a problem that the throttle valve is fully opened when the control unit malfunctions. One possible solution would be to use a system as disclosed in European Patent Provisional Publication No. 0089492, in which two throttle valves are provided in a series in an air intake passage, the first valve being driven by the accelerating pedal and the second valve being driven by a drive motor. Normally, the second throttle valve is fully opened and the first throttle valve is operated in accordance with operation of the accelerating pedal. In response to the requirement of the traction control, the second throttle valve is controlled to be closed so that the engine output is reduced irrespective of operation of the first throttle valve to allow the prevention of vehicle slipping. Although satisfactory for improving the reliability of the throttle valve control, this system results in a bulky throttle valve arrangement and difficulty is encountered to mount it on the motor vehicle.